Scanning ultrasonic inspection method and apparatus

ABSTRACT

A method and apparatus for testing for flaws by ultrasonic energy in which transducer elements are uniformly spaced in a row relative to a work piece and successive groups thereof are energized in a progressive manner along the row, each group being energized in the same manner so that successive foci are on a path on the outer surface of the work piece. Preferably, each transducer element emits a pulse of ultrasonic energy throughout a substantial angle towards the work piece and the pulses arrive substantially simultaneously and in phase at a point within the work piece.

Unite States atent Whittington [451 Sept. 26, 1972 [54] SCANNHNG ULTRASONIC INSPEGTION METHOD AND APPARATUS [72] Inventor: Keith Richard Whittington, Greatshelford, England [73] Assignee: T. I. (Group Services) Limited,

Birmingham, England [22] Filed: July 9, 1971 211 Appl.No.: 161,079

Related US. Application Data [63] Continuation-impart of Ser. No. 787,287, Nov.

26, 1968, abandoned.

[30] Foreign Application Priority Data 3,295,098 12/ 1966 Brightman et a1. ..340/5 3,324,452 6/1967 Brightman et a1. ..340/5 3,346,068 10/1967 Woods et al ..340/5 FOREIGN PATENTS OR APPLICATIONS 772,083 4/1957 Great Britain ..73/67.7 941,573 11/1963 Great Britain ..73/67.8 1,400,484 4/1965 France ..73/67.8

Primary Examiner-Richard C. Queisser Assistant ExaminerJohn P. Beauchamp Attorney-Scrivener, Parker, Scrivener & Clarke [5 7] ABSTRACT A method and apparatus for testing for flaws by ultrasonic energy in which transducer elements are uniformly spaced in a row relative to a work piece and successive groups thereof are energized in a progressive manner along the row, each group being energized in the same manner so that successive foci are on a path on the outer surface of the work piece. Preferably, each transducer element emits a pulse of ultrasonic energy throughout a substantial angle towards the work piece and the pulses arrive substantially simultaneously and in phase at a point within the work piece.

42 Claims, 10 Drawing Figures PATENTED SEP 2 s 1912 SHEET 1 BF 5 PATENTED E I972 3.693.415

saw 3 OF 5 PATENTEDSEPZB me 3 693 415 SHEET 6 UF 5 I SCANNING ULTRASONIC INSPECTION METHOD AND APPARATUS This is a continuation-in-part of the US. Pat. application Ser. No. 787,287 filed on Nov. 26, 1968, now abandoned, by Keith Richard Whittington.

This invention relates to a method of, and apparatus for, testing a test-piece with ultrasonic energy to enable it to be tested for flaws and, in particular, relates to the testing of tubes.

US. Pat. Nos. 3,021,706 and 3,052,115 both disclose ultrasonic testing system comprising ultrasonic transducer elements which each emit a directional or parallel beam of ultrasonic wave energy towards the test-piece to test a corresponding part of it when energized, and'which are sequentially energized one at a time so that a succession of such beams are formed to test a corresponding succession of different parts of the test-piece. The direction of the beam emitted by each transducer element and thus its angle of incidence on the test-piece is fixed and is determined by the orientation of the transducer element.

US. Pat. No. 3,166,731 discloses an ultrasonic testing system comprising ultrasonic transducer elements which each emit a directional beam of ultrasonic wave energy towards a test-piece and which are all arranged to emit their beams parallel to one another in the same direction to produce a resultant beam with a single wavefront. The shape of the wavefront and the direction of advance of the resultant beam are controlled by controlling the energizing sequence of the transducers. A linear array of transducers is used to produce a resultant beam with a planar wavefront which advances in a direction inclined to the component beams by energizing the transducers at regular intervals in a progressive manner along the array. The angle of inclination of the resultant beam is determined by the delay introduced between the energization of successive transducers, and can be varied thereby so that the beam scans through an angle relative to the test-piece.

US. Pat. No. 3,086,195 discloses an ultrasonic testing system comprising an ultrasonic transducer element formed from a plurality of co-planar parts, and means for applying oscillatory electrical energy to the parts with progressively different time delays so that ultrasonic wave energy from each forms a convergent beam which comes to a focus within the body of a testpiece. The illustrated transducer element comprises a plurality of concentric ring elements which are all energized to form a focus on the central axis of the transducer element at a distance from the plane of the transducer element determined by the time delay between successive concentric ring elements. This transducer element is placed in contact with the surface of the testpiece and the depth of the focus formed by the element within the body of the test-piece is varied by the said time delay. To scan the focus in a direction parallel to the surface of the test-piece the transducer element has to be moved across the surface of the test-piece. It is further suggested that the transducer element might take the form of a plurality of parallel strips to form a line focus.

A disadvantage common to all of these known systems is that they all lack flexibility, in that none will allow a beam of ultrasonic wave energy to be produced with any required angle of incidence at any point or region on the surface of the test-piece opposite the transducer elements. An object of the present invention is to overcome this disadvantage.

This is achieved according to the invention by arranging that individual transducer elements in a row can be energized so that they each emit a pulse of ultrasonic wave energy which overlaps with similarly produced pulses from neighboring transducer elements in a region of the test-piece which is to be tested, and successively energizing different predetermined groups of transducer elements from the row so that the transducer elements of each individual group emit pulses which arrive substantially in phase at a certain point in said region and create a focus of local maximum intensity at that point, and so that the successive foci so created are displaced relative to one another in directions parallel to said surface of the test-piece.

In this way successive focused beams of ultrasonic energy are formed by successive groups of transducer elements, the focus of each beam being spaced from the last so that an effective focus is created which scans parallel with the surface of the test-piece.

The focus at any point can be formed by pulses from any of a plurality of different groups of transducer elements, the mean angle of incidence of the ultrasonic beam incident at the focus for each group being different. Thus the mean angle of incidence at any point on the surface of the test-piece can be selected as required. This is particularly useful when the invention is applied to the testing of tubes and like articles, as it allows the mean angle of incidence of the focused beam to be maintained constant as the focus scans across it, so as to allow the maximum possible transfer of energy into the wall of the tube.

Preferably, the transducer elements in each individual group are energized so that they emit pulses which arrive substantially simultaneously as well as in phase at the focus. Relatively short pulses can then be employed more effectively to form foci.

In tube testing systems the row of transducer elements takes the form of a ring of uniformly spaced transducer elements which are adapted to receive the tube concentrically within it. Successive groups of transducer elements, each comprising a fixed number of neighboring transducer elements, are energized in the the ring and each group of transducer elements is preferably energizing the same manner comprising energizing successive neighboring transducer elements at regular intervals. Such a simple energizing sequence can be used to produce in phase foci within the tube wall by allowing for refraction of the ultrasonic energy at the outer tube surface as it enters the tube wall, such refraction giving an additional focusing effect.

Further, in such a tube testing system, the same transducer elements which are used to form the foci are also used to detect return echo signals from flaws in the region of each focus by selecting a particular one of the transducer elements from each group to act as a receiver for return echo pulses from flaws in the region of the focus formed by that group.

The invention will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1 shows a schematic diagram of tube testing apparatus according to one embodiment of the invention,

FIG. Za-l shows schematic diagrams of transducer elements suitable for use in the invention,

FIG. 3 shows an alternative form of control means for use with the apparatus of FIG. 1,

FIG. 4 shows how a ring of transducer elements can be energized to form a focus within the wall of a tube,

FIG. 5 shows how a transducer element which emits a parallel beam of energy forms an effective diverging beam within a tube wall, and

FIG. 6 shows a schematic diagram of apparatus according to an alternative embodiment of the invention.

The ultrasonic tube testing system of FIG. 1 comprises a circular array of uniformly spaced ultrasonic transducer elements E each of which, when energized, emits a pulse of ultrasonic energy which radiates as from a line source parallel with the central axis of the array throughout a substantial angle (1 towards the center of the array as shown in dashed outline in FIG. 1. Each transducer element is energized by being connected to a common power source P through an individual gate G which is controlled by a computor C. For simplicity the connections between the gates G, power source P and computer C are shown for only a few of the transducer elements in FIG. 1 though they apply equally to all of them. Further, the number of elements shown is very much smaller than the number which would usually be used in practice. The tube T to be tested is disposed within the array of transducer elements so as to be concentric with the array. The tube is then tested at individual points in turn around its circumference by operating the gates in a controlled manner so as to energize the transducer elements and form an ultrasonic focus of maximum intensity at each individual point in turn as described more fully below. Flaws at these individual points produce return echo signals which are detected by the same transducer elements E under the control of an analyzer unit A. In this way a region around the circumference of the tube T can be tested for flaws the whole length of the tube being tested by moving the tube T axially relative to the array so that successive portions along the length of the tube are tested.

The focus is formed at any time by pulses emitted from a predetermined number of transducer elements in an are such as the transducer elements E, to E, in FIG. 1. These transducer elements are energized in a predetermined sequence which proceeds from E, to E and which leaves appropriate time intervals between the energization of successive transducer elements so that some of the ultrasonic wave energy of each of the pulses emitted by the four transducer elements arrive simultaneously and in phase at the focal point F, on the surface of the tube T to give a maximum intensity at that point. The path of propagation of the energy from each transducer element which contributes to the focal point F, is shown in solid lines and it will be appreciated that the said appropriate time intervals between energization of successive transducer elements corresponds to the difference between the paths of propagation for successive transducer elements. A focus is similarly produced at the point P, a short time later by energizing the transducer elements E to E in exactly the same sequence as transducer elements E, to E Similarly, other points, each displaced relative to the last, are formed by energizing transducer elements E to E E to E and so on. In this way an effective focus is formed which progresses in steps around the circumference of the tube.

The formation of successive foci in this way is dependent on each transducer element emitting ultrasonic energy which is incident over a corresponding extensive region of the outer surface of the tube so that it can contribute energy to a focus anywhere within this region, and on the fact that a circumferentially extending area on the outer surface of the tube is exposed to energy from all of the transducer elements of each group so that they can all contribute energy to a focus anywhere within this area. The particular energizing sequence applied to the transducer elements in a group determines the position of the focus within said exposed area i.e., determines the mean angle of incidence of the energy forming the focus. Clearly, where as in the system of FIG. 1 the energizing sequence within each group of transducer elements remains constant the mean angle of incidence of the energy forming each focus remains constant and is preferably arranged to be slightly less than 30 so as to allow the maximum possible energy to pass into the tube wall in the form of shear waves and thereby give the maximum efficiency in the detection of flaws in the outer surface layer.

A standard tube testing programme is employed in the computer C which incorporates information as to the number of transducer elements in each group of transducer elements, the number of transducer elements by which each such group is displaced from the last to produce successive foci, and the energizing sequence required within each such group to produce a focus with a particular associated mean angle of incidence on the outer surface of a tube of a particular diameter. For tubes with different diameters within a set range of diameters the computer can adjust the energizing sequence required automatically. The energizing sequence employed is calculated using information as to the position of the point or line from which the transducer elements emit energy throughout an angle a. As described below the position of this line may be different from the actual position of the transducer element. The range of diameters of tubes which can be tested using a set array of transducers elements increases with increasing angles a due to the correspondingly increased circumferential area on the outer surface of the tube which is exposed to energy from all of the transducer elements in a group.

Each transducer element is energized by applying a sudden step voltage to it, the power source P providing the voltage and a gate G switching this voltage when operated by the computer C. As a result the transducer elements oscillate mechanically at their resonant ultrasonic frequency and thus create ultrasonic waves in the coupling medium between the transducers and the tube. These waves are only emitted for a short time, as the oscillations of the transducer elements are damped, the transducer elements typically emitting pulses of five cycles, each cycle being of exponentially decreasing amplitude over the preceding cycle. Ideally the transducer elements are all exactly the same so that they all have the same resonant frequency and the pulses of waves emitted by each group of four transducer elements and arriving simultaneously at the focal point are all in phase at the focal point. In this way the maximum possible intensity is obtained at the focal point. In practice, however, the transducer elements are usually all slightly different, thus the pulses although initially in phase may gradually get out of phase but as the pulses are short this will only have a slight effect on the intensity at the focal point.

The transducer elements are much smaller than those conventionally used in such ultrasonic testing systems and thus they require much less power to energize them. Because of this feature the gates G which have to switch these voltages and currents to energize the transducer elements may comprise a number of solid-state devices. The advantage of using such devices is that they are small and inexpensive, further they can perform the high speed switching which is involved in the energizing sequence applied to each group of transducer elements.

Another advantage of using small transducer elements is that a great number of them can be provided in the transducer array so that the distance between successive focal points of the beam on the tube T, such as F, and F is small. In this way the tube can be examined for faults around substantially the whole of its circumference.

Examples of suitable piezo-electric transducer elements which emit ultrasonic wave energy throughout a substantial angle are shown in FIG. 2. FIGS. 2a), 2b), and show crossections through curved transducer elements E which oscillate radially when energized so as to emit energy as if from a line source f. In FIG. 2a) the transducer is a cylindrical tube of piezo-electric material and in FIGS. 2b) and 2c) the transducers are sections from such a tube. Typically the tube has an outer diameter of 13 centimeters, is 0.05 centimeters thick and 0.6 centimeters long, has a natural oscillating frequency of 3MI-Iz, and emits ultrasonic energy throughout an angle of 22%, 30 or 45. FIGS. 2d) and 2e) show cross-sections through planar transducer elements E which emit energy from an edge or end face as if from a line source f, the elements being made to oscillate along a line parallel to the emitting edge or end face and being of a width substantially equal to half of the wavelength of the emitted ultrasonic wave energy. Typically, such planar transducer elements are 1 centimeter long and 3 millimeters wide. All of these transducer elements typically draw a current of 8 milliamperes when a voltage of 20 volts is applied to them.

Other transducer elements may be spherical transducer elements or transducer elements comprising a section of a sphere which emit energy as if from a point source. Also, planar transducer elements may be used which emit ultrasonic wave energy as if from a point or line source by providing either a divergent lens in front of each transducer element, or a pinhole which diffracts the wave energy passing through it. Another arrangement may comprise a planar transducer element with a tuned mechanical transformer connected to a flat face thereof which is oscillated by the transducer element to emit divergent wave energy.

An example of a system as shown in FIG. 1 which employs small transducer elements as described above comprises ninety such transducer elements equispaced in a circle at 4 spacings, the effective line sources f defined by the transducer elements all lying on a circle of diameter 15.2 centimeters. Groups of ten transducer elements at a time are energized to form each focus and successive groups are displaced progressively in one case by just one transducer element at a time so that ninety consecutive foci are formed around the circumference of the tube. The time intervals between the energization of successive transducer elements in a group are of the order of 0.1 to 1 micro-seconds. Alternatively, groups of more or less than 10 transducer elements may be energized and successive groups may be displaced by two or more transducer elements at a time so that less than foci are formed around the circumference of the tube.

The return echo from any flaw in the region of a focus is picked up by one or more of the transducer elements which was energized to form that focus. Thus, for each successive group of emitting transducer elements there isa corresponding group of receiving transducer elements within the emitting group. These groups of receiving transducer elements are selected under the control of the analyzer unit A, the latter comprising individual gates provided in output connections form the transducer elements so as to allow gating of electrical echo signals from any of the transducer elements. In FIG. 1 only a few of the connections between the analyzer unit A and transducer elements E are shown for clarity.

In the simple case in which just one transducer element at a time is used to receive echo signals, the analyzer A operates in step with the computer C to gate the output signal from successive individual transducer elements around the array. These successive transducer elements may be, for example, the transducer element E for the emitting group E to E the trans ducer element E, for the emitting group E to E the transducer element E for the emitting group E to E and so on.

If two or more transducer elements at a time are used to receive echo signals, the analyzer unit A operates in step with the computer C to gate the output signals from successive groups of two or more transducer elements, and also operates to allow for the fact that the output signals generated in the different transducer elements of a receiving group by the same return echo will be out of phase due to the different path lengths between these transducer elements and the corresponding focus. This last mentioned operation consists in introducing appropriate phase delays between the different output signals so that they are then in phase and reinforce one another to give a larger signal when added together. Clearly, the appropriate phase delay required between any two transducer elements in a receiving group will correspond to the time interval introduced between the energization of these same transducer elements when they are energized as part of the corresponding emitting group. In one example, the same four transducer elements such as E to E. which are used to form a focus can be used as the receiving transducer elements for return echoes from that focus. It is preferred to employ two or more receiving transducer elements at a time as the sensitivity of the system is improved as compared with a system which employs just one receiving transducer element at a time.

In an alternative embodiment of the invention the transducer elements E which emit energy to form successive foci are used exclusively for this purpose, and separate receiving transducer elements are provided between the transducer elements E, these receiving transducer elements being controlled by an analyzer unit in a similar manner to that described above.

In another alternative embodiment of the invention the apparatus shown schematically in FIG. 1 for sequentially energizing the transducer elements E is replaced by apparatus shown schematically in FIG. 3 and comprising a clock generator S which is connected to a multi-stage shift register R, all of the stages of the shift register being connected to each of the transducer elements through solid-state gates H which are under the control of acomputer C. For simplicity the stages of the shift register are only shown connected to one transducer element E although similar connections are made to all of them, and further a five stage register is shown. The computer C "calculates the energizing sequence required in a group of transducer elements and opens appropriate gates H to connect each transducer element in the group to an appropriate stage of the register so that when the clock generator is started the transducer elements in the group are energized in the correct sequence to form a focus at a required point. The clock is then stopped, a new focus calculated by the computer, a new pattern of gates opened and the clock started again. This arrangement can be used in testing systems as described above, a standard program being used in the computer for tube testing if required. An advantage of this arrangement over the illustrated arrangement, however, is that it is not dependent on the speed of the computer C for its proper functioning, whereas the illustrated arrangement is, and thus it can be used where a sufficiently fast computer is not available.

In the ultrasonic tube testing system described so far transducer elements within a group are sequentially energized so as to emit pulses of ultrasonic energy which arrive at a point simultaneously and in phase and form a focus of maximum intensity at that point. To produce this result time delays are introduced between the energization of successive transducer elements within the group and, as will be appreciated by considering the propagation paths shown in solid lines in FIG. 1 between the transducer elements E to E and F,, the successive time delays introduced between the transducer elements will necessarily all be different, decreasing throughout the energizing sequence. It is possible, however, to form a less intense focus, but one which may nevertheless still be suitable for testing purposes by introducing a suitable constant time interval between the energization of successive transducer elements. The less intense focus is produced because the pulses at the focus are then slightly out of phase.

A tube testing system which forms a less intense focus of the above kind is now described. This system comprises a circular array of uniformly spaced transducer elements which is arranged concentrically with the tube to be tested, as in the system shown in FIG. 1. The transducer elements are energized one at a time at regular intervals in a progressive manner around the array. Thus, if a transducer array like that shown in FIG. 1 is used, and the progress of energization is clockwise, then transducer element E is energized first, transducer element E, is energized a short time later, and then transducer element E is energized an equal time after that, and so on around the array. As a result the transducer elements emit pulses of ultrasonic waves which form a focus which moves in step with the energizing sequence in a clockwise direction around a circle within the array. The radius of the circle is determined by the time intervals between the energization of successive transducer elements and is suitable adjusted to bring the focus onto the surface of the tube. This radius, however, is preferably kept small compared with the radius of the array as the phase condition will then be almost satisfied and a more intense focus obtained than would be the case otherwise.

In energizing the transducer elements separate groups of transducer elements are not specifically selected but it will be appreciated that the focus is formed at any one time by the pulses emitted from just a' few of the more recently energized transducer elements'which themselves comprise a group, the oscillations of the transducer elements which were energized earlier having substantially died away. In this case where the progress of energization is clockwise the focus is displaced in a clockwise direction from a line joining the center of the array to the center of the are on which the transducer elements forming the focus lie. Thus the ultrasonic beam forming the focus is not incident normally on the tube and the condition that the angle of incidence should be that angle for the maximum transfer of energy into the wall of the tube is approached.

Return echo signals can be detected by the transducer elements E or separate individual transducer elements in the manner described above in connection with the in phase system of FIG. 1. However, because of the large number of transducer elements being energized in such a short time (typically, a complete energizing sequence running through all transducer elements takes micro-seconds or less) there is a lot of background noise in the reflected echo which lowers the sensitivity of the system. This background noise comprises pulses transmitted through the tube from the transducer elements on the opposite side of the array from the receiving transducers. This problem can be overcome by energizing only a proportion of the transducer elements, say 30, at a time with longer pulses between the energization of successive groups of 30 transducer elements. Successive groups of 30 transducer elements may include transducer elements from the group last energized. Obviously, a complete test cycle then takes longer but the sensitivity of the system is much improved.

The energizing sequence used in this system is simpler than that used in the in-phase systems, and thus simpler and less expensive energizing and control means can be used to perform it. The energizing and control means may, nevertheless, be similar to those described above but where a computer is used this need preciated, however, that in tube testing it may be necessary to form foci anywhere within the wall of the tube to test for flaws therein. This can be done quite readily without making any changes in the systems described above. However, in determining the energizing sequence which must be applied to a group of transducer elements to form a particular focus the effect of refraction of the ultrasonic energy as it passes into the wall of the tube must be taken into account.

FIG. 4 illustrates how ultrasonic pulses emitted by three transducer E to E in an array similar to that of FIG. 1 contribute energy to a focal point F within the wall of the tube T, the dashed lines showing the angular spread a of the energy emitted by each transducer element and the solid lines showing the path of propagation of the energy actually reaching the focus F. This diagram shows clearly how the energy is refracted at the outer surface of the tube, and also shows how this refraction causes an additional focusing effect in that it causes the energy producing the focus to become more convergent within the tube wall due to the progressive change in the angle of incidence of the energy producing the focus and the circular nature of the array of transducer elements E. This additional focusing effect is particularly important in that it can be used so that the path difference between the paths of propagation of energy reaching the focus from successive transducer elements is constant, thus allowing successive transducer elements in a group such as E to E to be energized in the simplest manner with equal intervals of time between the energizing of each without any substantial decrease in the intensity of the focus.

Another important feature which is noticeable in FIG. 4 is that when producing a focus within the tube wall two transducer elements such as IE and E, which do not emit energy which is incident over a common area at the surface of the tube can nevertheless con tribute energy to the same focus F within the tube wall due to refraction at the outer surface of the tube. Thus, the angular spread of the emitted energy is not so impoitant as it is when considering a focus at the surface of the tube. In fact, it is even possible to employ transducer elements such as shown in FIG. 5 which emit a parallel beam of ultrasonic energy. After refraction at the outer surface of the tube such a beam becomes divergent, effectively coming from a point or lines source E.

As has already been mentioned above the mean angle of incidence at the outer surface of the tube of that energy forming the focus is preferably kept constant at slightly less than 30 so as to allow the maximum possible energy to pass into the tube wall. The importance of this requirement can be more readily seen when a focus is formed within the tube wall proper.

The focus F formed by a group of transducer elements such as E, to E can be formed anywhere within the wall of the tube by appropriate choice of the energizing sequence applied to the transducer elements. Further, the focus can be formed either directly after the focused energy has been refracted or after it has been reflected internally once or more times from the surfaces of the tube. A succession of such foci can be formed one after the other around the tube by energizing successive overlapping groups of transducer elements such as E, to E to E and so on, the energizing sequence within each group remaining constant and comprising energizing successive transducer elements at regular intervals of time. Alternatively, a focus can be formed which scans continuously around the tube by energizing the transducer elements E E E E, and so on in a progressive manner around the tube with equal intervals of time between the energization of successive transducer elements and without any regard to selecting individual groups of transducer elements. The means employed to energize the transducer elements and to detect return echo signals from flaws in the region of the'focus may take the various forms already described above.

The apparatus illustrated schematically in FIG. 6 is generally suitable for controlling energization of a row of transducer elements so that successive similar groups of transducer elements are energized in a progressive manner along the row and successive transducer elements in each group are energized at regular intervals which remain unchanged from group to group. The apparatus further serves to control the detection of return echo signals by selecting a particular one of the transducer elements from each group to act as a receiver for return echo pulses from flaws in the re gion of the focus formed by that group. Basically, the apparatus operates by uniquely coding each transducer element and supplying a suitable sequence of coded pulses to energize them and to select them for receiving purposes. The illustrated system is specifically designed for use with a ring of ninety uniformly spaced transducer elements and serves to energize successive groups of 10 neighboring transducer elements which are each displaced by one transducer element from the last. The transducer elements are coded 0 to 89 progressively around the ring and a series of coded pulses 0 to 9, 1 to 10, 2 toll, and so on are used to energize the groups of correspondingly coded transducer elements. The coded pulses are produced by two binary counters 1 and 2 which are fed pulses from a variable frequency pulse oscillator 3 the frequency setting of which determines the length of the intervals between energization of successive transducer elements in each group, and the rate at which successive groups of transducer elements are energized is determined by the repetition frequency of a pulse generator 4.

Stop and start buttons 5 and 6 and a clear button 7 are used to control the apparatus. The clear button 7 is operated initially to reset both of the binary counters l and 2 to the zero state, it setting a bistable 8 and thereby causing the latter to supply a reset signal to both counters. The bistable 8 is then automatically reset itself by a reset signal which is generated by the counter 2 when in its zero state and which is supplied to the bistable 8 through a connection 9. After the clear button 7 has been operated the start button 5 can be operated to allow the supply of pulses from the oscillator 3 to the counters l and 2. The stop and start buttons control setting and rc-setting, respectively, of a bistable circuit 10 which has an output connection to the input of a gate 11. The gate 11 also has input connections from the oscillator 3 and generator 4. Operation of the start button 3 produces an output signal from the bistable 10 which allows the gate 11 to pass a coincidence pulse every time that the oscillator 3 and generator 4 produce output pulses simultaneously. This pulse sets two bistable circuits l2 and 13 causing the first to open a gate 14 so that it passes oscillator pulses from the oscillator 3 to the two binary counters l and 2, and causing the second to open transmitting gates 15 so that coded pulses corresponding to these input oscillator pulses are gated from the counter l. Successive coincidence pulses are produced in this way and cause the counter l to generate successive sets of coded pulses to energize groups of transducer elements until such time as the stop button is operated to reset the bistable l and close gate 11.

The required coding pattern of successive output pulses from the counter l is provided by arranging that the counter l is a binary counter with a maximum count capacity of 89, and that the counter 2 is a binary counter with a maximum count capacity of ninety and controls the gate 14. After the first coincidence pulse is passed by the gate 11 both counters count up from zero together and in each of the states 0 to 9 the counter 1 passes a correspondingly binary coded output pulse through a binary/decimal converter unit 16 and the open transmitting gates 15 to a decoder unit 17. On the count of 10 the counter 2 generates a reset signal which is supplied through a connection 18 to the bistable 13 resetting the latter and thus closing the transmitting gates 15. Thus, as both counts count on past nine none of the coded output signals, 10 onwards, from the counter 1 pass to the decoder 17. Eventually, the nintieth oscillator pulse causes the counter 1 to reset itself to zero automatically and puts the counter 2 into its ninetieth state. The next oscillator pulse thereafter puts the counter 1 into its one state and causes the counter 2 to reset itself to zero automatically and to pass a reset signal through a connection 19 to bistable 12 which responds by closing the gate 14 to stop any further oscillator pulses passing to the counters.

The two counters are now out of step with one another by one count and when the second coincidence pulse opens the gate 14, counter 1 counts up from 1 to 10 while counter 2 counts up from 0 to 9. Counter 2 again closes the transmitting gates 15 on the count of 10 which stops the coded pulses 11 onwards from passing between the counter l and the decoder 17. Further, both counters are eventually reset to zero but because they are out of step by one count in counting up from zero, counter 1 has reset and reached the count of two by the time counter 2 has reset to zero and closed gate 14. Thus, on receipt of the third coincidence pulse the two counters count up from zero out of step with one another by two counts and the transmitting gates 15 passes the coded pulses 2 to 11 before they are again closed by counter 2 on the count of ten and the counter l resets and finally reaches the count of three before counter 2 resets to zero. Similarly, the fourth coincidence pulse causes the transmitting gates 15 to pass the coded pulses 3 to 12, the fifth coincidence pulse causes the transmitting gates 15 to pass the coded pulses 4 to 13, and so on. The 81st, 82nd, and 83rd coincidence pulses will cause the transmitting gates 15 to pass the coded pulses 80 to 89, 81 to 0, and 82 to 1 respectively, and eventually the 91st coincidence pulse will cause the transmitting gates 15 to pass the coded pulses 0 to 9 again.

Successive trains of ten coded pulses are therefore fed to the decoder unit 17 which is adapted to respond to each coded pulse in turn by opening a correspondingly coded gate to energize an associated transducer element in the ring so that successive corresponding groups of transducer elements are energized The time intervals between successive pulses in each train is determined by the repetition frequency of the oscillator 3 and is preset so that the group of transducer elements energized by each train of coded pulses emits ultrasonic energy to form a focus, at a particular point. The time interval between each train of pulses is determined by the repetition frequency of the generator 4 and is preset to define a receiving interval after each energizing interval during which a particular one of the transducer elements of the group last energized is used to receive any return echo signals from flaws.

The receiving interval after each energizing interval is triggered automatically by the same coincidence pulse passed by the gate 11 to start that energizing interval. This pulse operates a first monostable circuit 20 which, after a set delay ending after counter 2 resets to zero, operates a second monostable circuit 21. The operated monostable 21 remains in its astable state for a set time ending before the gate 11 passes the next coincidence pulse to start the next energizing interval. While in the astable state it supplies an output signal to receiving gates 22 and thereby holds them open to allow a coded pulse derived from the output of the counter l to pass from a binary/decimal converter unit 23 to a decoder unit 24. The receiving gates 22, converter 23 and decoder 24 are all similar to the transmitting gates 15, converter 16 and decoder 17 and they operate in a similar manner except that they handle only the one coded pulse derived from the output of the counter l and that the decoder unit 24 operates a correspondingly coded gate to gate electrical echo pulses from an associated transducer element rather than to energize that element. The coded pulse may correspond directly to the state of the counter 1 at the end of the energizing interval this pulse being fed to the converter 23 through an addition unit 25 which is preset so as to have no effect on the pulse. Altematively, however, the addition unit 25 can be preset so as to add any number from one to eight to the coding number of the pulse from the counter 1. By these means it is possible to arrange that any one of the last nine of the transducer elements which are energized in an energizing interval can be used as a receiver in the following receiving interval.

If the addition unit is set to zero the coding of the pulse fed to the decoder 24 will be 1 after the first coincidence pulse, 2 after the second coincidence pulse, and so on. Clearly, if the addition unit is set to add one or any other number up to eight the coding of all of these pulses will be increased by one or the appropriate number.

The electrical echo pulses gated from the transducer element in each receiving interval are used to produce a visual display on a cathode ray tube 26, the pulses being fed to an integrating and amplifier unit 27 which feeds the resulting output to the Y-plates of the cathode ray tube 26. The Y-deflection corresponding to each of the ninety different receiving intervals is displayed at a different X-coordinate on the screen of the cathode ray tube by using the coincidence pulses from the gate 11 to drive a staircase generator 28 with an output to the X-plates of the cathode ray tube. Each coincidence pulse causes the generator 28 to deflect the beam one step in the X-direction and the generator 28 allows 89 such deflections progressively across the screen before returning to the start again. Also, the cathode of the cathode ray tube 26 is fed a bright up pulse for each separate Y-deflection which is derived from the output of the monostable 21 and which is delayed by a monostable 29 so as to coincide with the corresponding Y-deflection. The display on the screen as shown at 30, therefore, comprises a succession of 90 horizontally spaced vertical lines each proportional to the energy of the return echo signals from a different focus. 7' V In the system illustrated in FIG. 6, the oscillator 3, generator 4, two binary counters l and 2, the decoder 17 and intermediate gates and bistable circuits all correspond to the computer C in the system illustrated in FIG. 1; and the other monostable circuits, decoder 24 and cathode ray tube 26 correspond to the analyzer unit A in the system of FIG. 1, where the analyzer unit A is coupled with the computer C. It will be appreciated, however, that the system of FIG. 6 is only suitable for testing tubes or like cylindrical test-pieces whereas the system of FIG. 1 can be used for testing test-pieces other than tubes if the computer C is programmed appropriately. Further, it will be appreciated that the transducer array shown in FIG. 1 need not necessarily be circular and may be linear in an altemative embodiment of the invention.

Further, in other alternative embodiments of the testing systems according to the invention, the single ring of transducer elements as shown in the illustrated embodiments may be supplemented by further rings which form a cylindrical array of transducer elements. The effective focus can then be scanned along the tube as well as around it thus enabling transverse flaws (i.e., flaws perpendicular to the axis of the tube) to be detected.

In yet other alternative embodiments of the invention instead of arranging the transducer elements in an array which is spaced from the surface of the test-piece, the transducer elements may be arranged in contact with the surface of the test-piece so as to transmit ultrasonic energy directly into the latter. In such embodiments there will be no refraction of the transmitted energy at the surface and foci will be formed directly as shown in FIG. 1.

I claim:

l. A method of testing a test-piece for flaws with ultrasonic energy comprising the steps of positioning the test-piece relative to a row of individually energizable transducer elements so that the transducer elements lie opposite a surface of the test-piece and each emit a pulse of ultrasonic energy towards said surface when energized, which pulse overlaps with similarly produced pulses of energy from neighboring transducer elements in a region of the test-piece which is to be tested; successively energizing different predetermined groups of transducer elements from the row so that the transducer elements of each individual group are energized in a predetermined timing sequence so as to emit pulses which arrive substantially in phase at a certain point in said region and create a focus of local maximum intensity at that point, and so that the successive foci so created are displaced relative to one another in directions parallel to said surface of the test-piece; and detecting ultrasonic echo pulses from any flaws in the region of each focus using transducer elements in which said echo pulses generate electrical echo signals which are then processed to give an indication of the presence of a flaw.

2. A method as claimed in claim 1 in which the transducer elements of each individual group are energized in a predetermined timing sequence so as to emit pulses which arrive substantially simultaneously and in phase at a certain point in said region and create a focus of local maximumintensity at that point.

3 A method as claimed in claim 2 in which the trans ducer elements are such that when energized they each emit a pulse of ultrasonic waves throughout a substantial angle towards the test-piece.

4. A method as claimed in claim 3 in which each of said groups of transducer elements is selected to include some of the transducer elements of the group last energized.

5. A method as claimed in claim 4 in which the testpiece is positioned relative to a row of individually energizable and uniformly spaced transducer elements, and in which each group of transducer elements is selected so as to consist of a predetermined constant number of neighboring transducer elements, successive groups of transducer elements being selected to have a predetermined constant number of transducer elements common to one another.

6. A method as claimed in claim 5 in which each group of transducer elements is energized in exactly the same predetermined timing sequence.

7. A method as claimed in claim 1 in which the transducer elements which are energized to emit ultrasonic pulses and create foci are also used to receive the ultrasonic echo pulses.

8. A method as claimed in claim 7 in which one or more of the transducer elements of each group which are energized to form a focus are used to receive ultrasonic echo pulses from any flaws in the region of that focus.

9. A method as claimed in claim 8 in which two or more of the transducer elements of each group which are energized to form a focus are used to receive ultrasonic echo pulses from any flaws in the region of that focus, and in which phase delays are introduced between the corresponding electrical signals generated in these transducer elements by the echo pulses so that they are in phase and reinforce one another when added together subsequently.

10. A method as claimed in claim 1 in which two or more transducer elements are used to receive the echo pulses from each focus, and in which phase delays are introduced between the corresponding electrical signals generated in these transducer elements by the echo pulses so that they are in phase and reinforce one another when added together subsequently.

11. A method of testing a cylindrical tube for flaws with ultrasonic energy comprising the steps of positioning the tube concentrically within a ring of individually energizable and uniformly spaced transducer elements so that each of the transducer elements emits a pulse of ultrasonic energy towards the center of the tube when energized which overlaps with similarly produced pulses of energy from neighboring transducer elements in a region of the tube which is to be tested; successively energizing different predetermined groups of transducer elements from the ring so that the transducer elements of each individual group are energized in a predetermined timing sequence so as to emit pulses which arrive substantially in phase at a certain point in said region and create a focus of local maximum intensity at that point, and so that the successive foci so created are displaced relative to one another circumferentially around the tube in a progressive manner; and detecting ultrasonic echo pulses from any flaws in the region of each focus using transducer elements in which said echo pulses generate electrical signals which are then processed to give an indication of the presence of a flaw.

12. A method as claimed in claim 11 in which the transducer elements are such that when energized they each emit a pulse of ultrasonic waves throughout a substantial angle towards the test-piece.

13. A method as claimed in claim 12 in which the transducer elements of each individual group are energized in a predetermined timing sequence so as to emit pulses which arrive substantially simultaneously and in phase at a certain point in said region and create a focus of local maximum intensity at that point.

14. A method as claimed in claim 13 in which each group of transducer elements is selected so as to consist of a predetermined constant number of neighboring transducer elenients, and in which each group of transducer elements is energized in exactly the same predetermined timing sequence.

15. A method as claimed in claim 14 in which successive groups of transducer elements are selected to have a predetermined constant number of transducer elements in common with one another.

16. A method as claimed in claim 15 in which the transducer elements of each group are energized in a progressive manner with equal intervals of time between the energization of successive neighboring transducer elements.

17. A method as claimed in claim 12 in which transducer elements are energized one at a time in a progressive manner around the ring so that there are equal intervals of time between the energization of successive transducer elements, and the transducer elements emit pulses which form a succession of foci.

18. A method as claimed in claim 17 in which all of the transducer elements are energized in the said progressive manner around the ring.

19. A method as claimed in claim 11 in which the transducer elements which are energized to emit ultrasonic pulses and create foci are also used to receive the ultrasonic echo pulses.

20. A method as claimed in claim 19 in which one or more of the transducer elements of each group which are energized to form a focus are used to receive ultrasonic echo pulses from any flaws in the region of that focus.

21. A method as claimed in claim 20 in which two or more of the transducer elements of each group which are energized to form a focus are used to receive ultrasonic echo pulses from any flaws in the region of that focus, and in which phase delays are introduced between the corresponding electrical signals generated in these transducer elements by the echo pulses so that they are in phase and reinforce one another when added together subsequently.

22. A method as claimed in claim 11 in which two or more transducer elements are used to receive the echo pulses from each focus, and in which phase delays are introduced between the corresponding electrical signals generated in these transducer elements by the echo pulses so that they are in phase and reinforce one another when added together subsequently.

23. Apparatus for testing a test-piece for flaws with ultrasonic energy comprising a row of separate transducer elements arranged opposite a surface of the testpiece each of which is adapted to emit a pulse of ultrasonic energy towards said surface when energized which overlaps with similarly produced pulses of energy from neighboring transducer elements in a region of the test-piece which is to be tested; energizing means to energize each of said transducer elements individually; control means which is adapted to control said energizing means so that the latter energizes each of a succession of different predetermined groups of said transducer elements in a predetermined manner such that the transducer elements of each individual group emit pulses which arrive in phase at a certain point in said region and create a focus of local maximum intensity at that point, and further such that the successive foci so created are displaced relative to one another in directions parallel to said surface of the test-piece; transducer elements which receive ultrasonic echo pulses from any flaws in the region of each focus and which generate corresponding electrical echo signals; and processing means which gates said electrical echo signals and which is adapted to give an indication of the presence of a flaw.

24. Apparatus as claimed in claim 23 in which the energizing means comprises an electrical power source and individual switching devices connected between each transducer element and said source, each of said switching devices being under the control of said control means and being operative to cause said source to energize the associated transducer element.

25. Apparatus as claimed in claim 24 in which said electrical power source is a d.c. voltage supply and in which each of the switching devices is a logic gate.

26. Apparatus as claimed in claim 24 in which the control means comprises a computer.

27. Apparatus as claimed in claim 24 in which each of said individual switching devices is uniquely coded and in which the control means comprises a coded pulse generator which produces a predetermined sequence of coded pulses which are fed to said switching devices to operate the corresponding ones and thereby energize the mociated transducer elements.

28. Apparatus as claimed in claim 27 in which said coded pulse generator includes a binary counter with an input from a pulse oscillator and with output connections which deliver said coded pulses.

29. Apparatus as claimed in claim 27 in which said transducer elements which receive ultrasonic echo pulses are the same transducer elements which are energized to produce said foci, and in which said processing means comprises individual switching devices associated with each of the transducer elements and which are each uniquely coded and are controlled to gate electrical echo signals by coded pulses from said coded pulse generator.

30. Apparatus as claimed in claim 23 in which the transducer elements are such as to emit energy throughout a wide angle towards said surface when energized.

31. Apparatus for testing a cylindrical tube for flaws with ultrasonic energy comprising a ring of uniformly spaced transducer elements arranged concentrically about the tube each of which is adapted to emit a pulse of ultrasonic energy towards the center of the tube when energized which overlaps with similarly produced pulses of energy from neighboring transducer elements in a region of the tube which is to be tested; energizing means to energize each of said transducer elements individually; control means which is adapted to control said energizing means so that the latter energizes each of a succession of different predetermined groups of said transducer elements in a predetermined manner such that the transducer elements of each individual group emit pulses which arrive in phase at a certain point in said region and create a focus of local maximum intensity at that point, and further such that the successive foci so created are displaced relative to one another in directions parallel to said surface of the tube; transducer elements which receive ultrasonic echo pulses from any flaws in the region of each focus and which generate corresponding electrical echo signals; and processing means which gates said electrical echo signals and which is adapted to give an indication of the presence of a flaw.

32. Apparatus as claimed in claim 31 in which the transducer elements are such as to emit energy throughout a wide angle towards said surface when ener2ized.

33. Apparatus as claimed in claim 32 in which the control means is adapted to cause energization of successive groups of transducer elements, each consisting of a predetermined constant number of neighboring transducer elements, in the same predetermined manner.

34. Apparatus as claimed in claim 33 in which the control means is adapted to cause energization of successive transducer elements in each group at regular intervals.

35. Apparatus as claimed in claim 34 in which said energizing means comprises an electrical power source and individual switching devices connected between each transducer element and said source, each of said switching devices being under the control of said control means and being operative to cause said source to energize the associated transducer element.

36. Apparatus as claimed in claim 35 in which each of said individual switching devices is uniquely coded and in which the control means comprises a coded pulse generator which produces a predetermined sequence of coded pulses which are fed to said switching devices to operate the corresponding ones and thereby energize the associated transducer elements.

37. Apparatus as claimed in claim 36 in which said coded pulse generator includes a binary counter with an input from a pulse oscillator and W1 output connections which deliver said coded pulses.

38. Apparatus as claimed in claim 37 in which said transducer elements which receive ultrasonic echo pulses are the same transducer elements which are energized to produce said foci, and in which said processing means comprises individual switching devices associated with each of the transducer elements and which are each uniquely coded and are controlled to gate electrical echo signals by coded pulses from said coded pulse generator.

39. Apparatus as claimed in claim 38 in which said binary counter of the coded pulse generator operates cyclically, counting up from a progressively differing count during successive cycles and resetting to zero automatically and counting up to the next starting count during each cycle.

40. Apparatus as claimed in claim 39 in which the state of said binary counter at the end of each cycle is used to produce a corresponding coded pulse to gate the electrical echo signal from a transducer element.

41. Apparatus as claimed in claim 40 in which successive gated electrical echo signals are used to produce successive horizontally spaced vertical deflections in a cathode ray tube display.

42. Apparatus as claimed in claim 32 in which the control means is adapted to cause energization of all of the transducer elements one at a time in a progressive manner around the ring so that there are equal intervals of time between the energization of successive transducer elements. 

1. A method of testing a test-piece for flaws with ultrasonic energy comprising the steps of positioning the test-piece relative to a row of individually energizable transducer elements so that the transducer elements lie opposite a surface of the test-piece and each emit a pulse of ultrasonic energy towards said surface when energized, which pulse overlaps with similarly produced pulses of energy from neighboring transducer elements in a region of the test-piece which is to be tested; successively energizing different predetermined groups of transducer elements from the row so that the transducer elements of each individual group are energized in a predetermined timing sequence so as to emit pulses which arrive substantially in phase at a certain point in said region and create a focus of local maximum intensity at that point, and so that the successive foci so created are displaced relative to one another in directions parallel to said surface of the test-piece; and detecting ultrasonic echo pulses from any flaws in the region of each focus using transducer elements in which said echo pulses generate electrical echo signals which are then processed to give an indication of the presence of a flaw.
 2. A method as claimed in claim 1 in which the transducer elements of each individual group are energized in a predetermined timing sequence so as to emit pulses which arrive substantially simultaneously and in phase at a certain point in said region and create a focus of local maximum intensity at that point.
 3. A method as claimed in claim 2 in which the transducer elements are such that when energized they each emit a pulse of ultrasonic waves throughout a substantial angle towards the test-piece.
 4. A method as claimed in claim 3 in which each of said groups of transducer elements is selected to include some of the transducer elements of the group last energized.
 5. A method as claimed in claim 4 in which the test-piece is positioned relative to a row of individually energizable and uniformly spaced transducer elements, and in which each group of transducer elements is selected so as to consist of a predetermined constant number of neighboring transducer elements, successive groups of transducer elements being selected to have a predetermined constant number of transducer elements common to onE another.
 6. A method as claimed in claim 5 in which each group of transducer elements is energized in exactly the same predetermined timing sequence.
 7. A method as claimed in claim 1 in which the transducer elements which are energized to emit ultrasonic pulses and create foci are also used to receive the ultrasonic echo pulses.
 8. A method as claimed in claim 7 in which one or more of the transducer elements of each group which are energized to form a focus are used to receive ultrasonic echo pulses from any flaws in the region of that focus.
 9. A method as claimed in claim 8 in which two or more of the transducer elements of each group which are energized to form a focus are used to receive ultrasonic echo pulses from any flaws in the region of that focus, and in which phase delays are introduced between the corresponding electrical signals generated in these transducer elements by the echo pulses so that they are in phase and reinforce one another when added together subsequently.
 10. A method as claimed in claim 1 in which two or more transducer elements are used to receive the echo pulses from each focus, and in which phase delays are introduced between the corresponding electrical signals generated in these transducer elements by the echo pulses so that they are in phase and reinforce one another when added together subsequently.
 11. A method of testing a cylindrical tube for flaws with ultrasonic energy comprising the steps of positioning the tube concentrically within a ring of individually energizable and uniformly spaced transducer elements so that each of the transducer elements emits a pulse of ultrasonic energy towards the center of the tube when energized which overlaps with similarly produced pulses of energy from neighboring transducer elements in a region of the tube which is to be tested; successively energizing different predetermined groups of transducer elements from the ring so that the transducer elements of each individual group are energized in a predetermined timing sequence so as to emit pulses which arrive substantially in phase at a certain point in said region and create a focus of local maximum intensity at that point, and so that the successive foci so created are displaced relative to one another circumferentially around the tube in a progressive manner; and detecting ultrasonic echo pulses from any flaws in the region of each focus using transducer elements in which said echo pulses generate electrical signals which are then processed to give an indication of the presence of a flaw.
 12. A method as claimed in claim 11 in which the transducer elements are such that when energized they each emit a pulse of ultrasonic waves throughout a substantial angle towards the test-piece.
 13. A method as claimed in claim 12 in which the transducer elements of each individual group are energized in a predetermined timing sequence so as to emit pulses which arrive substantially simultaneously and in phase at a certain point in said region and create a focus of local maximum intensity at that point.
 14. A method as claimed in claim 13 in which each group of transducer elements is selected so as to consist of a predetermined constant number of neighboring transducer elements, and in which each group of transducer elements is energized in exactly the same predetermined timing sequence.
 15. A method as claimed in claim 14 in which successive groups of transducer elements are selected to have a predetermined constant number of transducer elements in common with one another.
 16. A method as claimed in claim 15 in which the transducer elements of each group are energized in a progressive manner with equal intervals of time between the energization of successive neighboring transducer elements.
 17. A method as claimed in claim 12 in which transducer elements are energized one at a time in a progressive manner around the ring so that there are equal intervals of time between the energization of successive transducer elements, and the Transducer elements emit pulses which form a succession of foci.
 18. A method as claimed in claim 17 in which all of the transducer elements are energized in the said progressive manner around the ring.
 19. A method as claimed in claim 11 in which the transducer elements which are energized to emit ultrasonic pulses and create foci are also used to receive the ultrasonic echo pulses.
 20. A method as claimed in claim 19 in which one or more of the transducer elements of each group which are energized to form a focus are used to receive ultrasonic echo pulses from any flaws in the region of that focus.
 21. A method as claimed in claim 20 in which two or more of the transducer elements of each group which are energized to form a focus are used to receive ultrasonic echo pulses from any flaws in the region of that focus, and in which phase delays are introduced between the corresponding electrical signals generated in these transducer elements by the echo pulses so that they are in phase and reinforce one another when added together subsequently.
 22. A method as claimed in claim 11 in which two or more transducer elements are used to receive the echo pulses from each focus, and in which phase delays are introduced between the corresponding electrical signals generated in these transducer elements by the echo pulses so that they are in phase and reinforce one another when added together subsequently.
 23. Apparatus for testing a test-piece for flaws with ultrasonic energy comprising a row of separate transducer elements arranged opposite a surface of the test-piece each of which is adapted to emit a pulse of ultrasonic energy towards said surface when energized which overlaps with similarly produced pulses of energy from neighboring transducer elements in a region of the test-piece which is to be tested; energizing means to energize each of said transducer elements individually; control means which is adapted to control said energizing means so that the latter energizes each of a succession of different predetermined groups of said transducer elements in a predetermined manner such that the transducer elements of each individual group emit pulses which arrive in phase at a certain point in said region and create a focus of local maximum intensity at that point, and further such that the successive foci so created are displaced relative to one another in directions parallel to said surface of the test-piece; transducer elements which receive ultrasonic echo pulses from any flaws in the region of each focus and which generate corresponding electrical echo signals; and processing means which gates said electrical echo signals and which is adapted to give an indication of the presence of a flaw.
 24. Apparatus as claimed in claim 23 in which the energizing means comprises an electrical power source and individual switching devices connected between each transducer element and said source, each of said switching devices being under the control of said control means and being operative to cause said source to energize the associated transducer element.
 25. Apparatus as claimed in claim 24 in which said electrical power source is a d.c. voltage supply and in which each of the switching devices is a logic gate.
 26. Apparatus as claimed in claim 24 in which the control means comprises a computer.
 27. Apparatus as claimed in claim 24 in which each of said individual switching devices is uniquely coded and in which the control means comprises a coded pulse generator which produces a predetermined sequence of coded pulses which are fed to said switching devices to operate the corresponding ones and thereby energize the associated transducer elements.
 28. Apparatus as claimed in claim 27 in which said coded pulse generator includes a binary counter with an input from a pulse oscillator and with output connections which deliver said coded pulses.
 29. Apparatus as claimed in claim 27 in which said transducer elements which receive ultrasonic echo pulses are the same tRansducer elements which are energized to produce said foci, and in which said processing means comprises individual switching devices associated with each of the transducer elements and which are each uniquely coded and are controlled to gate electrical echo signals by coded pulses from said coded pulse generator.
 30. Apparatus as claimed in claim 23 in which the transducer elements are such as to emit energy throughout a wide angle towards said surface when energized.
 31. Apparatus for testing a cylindrical tube for flaws with ultrasonic energy comprising a ring of uniformly spaced transducer elements arranged concentrically about the tube each of which is adapted to emit a pulse of ultrasonic energy towards the center of the tube when energized which overlaps with similarly produced pulses of energy from neighboring transducer elements in a region of the tube which is to be tested; energizing means to energize each of said transducer elements individually; control means which is adapted to control said energizing means so that the latter energizes each of a succession of different predetermined groups of said transducer elements in a predetermined manner such that the transducer elements of each individual group emit pulses which arrive in phase at a certain point in said region and create a focus of local maximum intensity at that point, and further such that the successive foci so created are displaced relative to one another in directions parallel to said surface of the tube; transducer elements which receive ultrasonic echo pulses from any flaws in the region of each focus and which generate corresponding electrical echo signals; and processing means which gates said electrical echo signals and which is adapted to give an indication of the presence of a flaw.
 32. Apparatus as claimed in claim 31 in which the transducer elements are such as to emit energy throughout a wide angle towards said surface when ener2ized.
 33. Apparatus as claimed in claim 32 in which the control means is adapted to cause energization of successive groups of transducer elements, each consisting of a predetermined constant number of neighboring transducer elements, in the same predetermined manner.
 34. Apparatus as claimed in claim 33 in which the control means is adapted to cause energization of successive transducer elements in each group at regular intervals.
 35. Apparatus as claimed in claim 34 in which said energizing means comprises an electrical power source and individual switching devices connected between each transducer element and said source, each of said switching devices being under the control of said control means and being operative to cause said source to energize the associated transducer element.
 36. Apparatus as claimed in claim 35 in which each of said individual switching devices is uniquely coded and in which the control means comprises a coded pulse generator which produces a predetermined sequence of coded pulses which are fed to said switching devices to operate the corresponding ones and thereby energize the associated transducer elements.
 37. Apparatus as claimed in claim 36 in which said coded pulse generator includes a binary counter with an input from a pulse oscillator and with output connections which deliver said coded pulses.
 38. Apparatus as claimed in claim 37 in which said transducer elements which receive ultrasonic echo pulses are the same transducer elements which are energized to produce said foci, and in which said processing means comprises individual switching devices associated with each of the transducer elements and which are each uniquely coded and are controlled to gate electrical echo signals by coded pulses from said coded pulse generator.
 39. Apparatus as claimed in claim 38 in which said binary counter of the coded pulse generator operates cyclically, counting up from a progressively differing count during successive cycles and resetting to zero automatically and counting up to the next starting count during eAch cycle.
 40. Apparatus as claimed in claim 39 in which the state of said binary counter at the end of each cycle is used to produce a corresponding coded pulse to gate the electrical echo signal from a transducer element.
 41. Apparatus as claimed in claim 40 in which successive gated electrical echo signals are used to produce successive horizontally spaced vertical deflections in a cathode ray tube display.
 42. Apparatus as claimed in claim 32 in which the control means is adapted to cause energization of all of the transducer elements one at a time in a progressive manner around the ring so that there are equal intervals of time between the energization of successive transducer elements. 